Irritable Bowel Syndrome

The irritable bowel syndrome (IBS) is the most common of all digestive disorders, affecting nearly everyone at one time or another and accounting for up to 50% of patients referred to a gastroenterology practice. Although characterized as a disorder of bowel motility, in many patients it usually is an exaggeration of normal physiologic responses and possibly heightened perception of pain.

Numerous terms have been used to describe the syndrome (Table 33-1). Irritable bowel syndrome seems to be the most appropriate. Terms that include the words colon or colitis are inaccurate because the condition is not limited to the colon, and inflammation is not a feature. Furthermore, use of the term colitis leads to confusion with ulcerative colitis and conveys an inaccurate impression to the patient.

In many patients IBS may be characterized as diarrhea predominant (IBS-D), constipation predominant (IBS-C), and for some patients, an alternation of diarrhea with constipation (alternators).

I. PATHOGENESIS.

The causes and pathogenesis of IBS remain obscure. Nevertheless, clinical and laboratory evidence indicate that it most likely is a disorder of bowel motility and increased sensory perception of pain. Constipation and abdominal cramps are prominent complaints of many patients with IBS-C. These symptoms could be explained on the basis of hypertonic segmental contractions, which would slow transit by increasing the resistance to passage of feces. On the other hand, it is possible that patients with diarrhea (IBS-D) have a hypomotile bowel, which would decrease resistance to passage of feces, or that they simply have an increase in peristaltic contractions.

A. Myoelectric activity of the colon

is composed of slow waves and spike potentials superimposed on the slow waves. In healthy individuals, slow-wave frequency ranges from 6 to 10 cycles per minute, although rates of 3 cycles per minute occur some of the time. The superimposed spike potentials take the form of short spike bursts and long spike bursts. The short spike bursts are less than 5 seconds and occur at the same time as the slow waves, resulting in muscular contractions of the same frequency as the slow waves. On the other hand, long spike bursts last from 15 seconds to several minutes and produce sustained contractions. Abnormalities in colonic myoelectric activity have been described in patients with IBS, but the findings have been inconsistent and, thus far, of no practical clinical use.

B. Intestinal motor activity.

In patients with IBS the increase in colonic motor activity that normally occurs after eating is blunted but continues longer than in asymptomatic individuals and may even become stronger. Emotional stress also induces colonic motor activity, both in healthy individuals and in patients with IBS, but it is possible that symptoms are perceived to a greater degree in patients with IBS. Balloon distention of the rectosigmoid colon in patients with IBS causes spastic contractions of greater amplitude than in asymptomatic subjects. Furthermore, there is evidence that patients with IBS who complain of gaseous distention and abdominal cramps cannot tolerate quantities of small-bowel intraluminal gas that are easily tolerated by healthy individuals (see Chapter 34).

TABLE 33-1 Synonyms for Irritable Bowel Syndrome

Irritable colon syndrome	Splenic flexure syndrome
Spastic bowel syndrome	Functional bowel disease
Spastic colitis	Psychophysiologic bowel disease
Mucous colitis	Nervous bowel

II. DIAGNOSIS

A. Clinical presentation

1. Symptoms.

Patients with IBS typically complain of crampy abdominal pain, diarrhea, or constipation. In some patients, chronic constipation is punctuated by brief episodes of diarrhea. A minority of patients have only diarrhea. Symptoms usually have been present for months to years, and it is common for patients with IBS to have consulted several physicians about their complaints and to have undergone one or more gastrointestinal evaluations.

2. Timing of symptoms.

The patient may be able to correlate symptoms with emotional stress, but often such a relation is not evident or becomes apparent only after careful questioning as the physician becomes acquainted with the patient. If abdominal cramps are a feature, they often are relieved temporarily by defecation. Bowel movements may be clustered in the morning or may occur throughout the day, but rarely is the patient awakened at night. Stools may be accompanied by an excessive amount of mucus, but blood is not present unless there is incidental bleeding from hemorrhoids.

3. The differential diagnosis

is broad, including most disorders that cause diarrhea and constipation (see Chapters 28, 29, 30, and 35). However, there are several features that suggest the diagnosis of IBS (Table 33-2). Several organic disorders may mimic IBS and, in fact, may be unrecognized for years in patients who mistakenly have been diagnosed as having IBS. Patients with lactose intolerance typically have postprandial diarrhea associated with crampy pain (see Chapter 31

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